In recent years, along with expansion of demands related to communication, methods for, for example, increasing the numbers of optical fiber cores, increasing the optical signal capacity per wavelength, increasing the numbers of wavelength division multiplexing (WDM) channels, and so on to thereby expand the transfer capacity have been sought for. However, since the cost of laying optical fibers, and the like are high, attempts are being made to expand the transfer capacity mainly by increasing the optical signal capacity or by increasing the numbers of WDM channels, without increasing the numbers of optical fiber cores.
Such an optical transfer system uses digital coherent optical receivers on receiving-side transfer devices. Such a receiver uses local emission light to optically demodulate signal light from reception light. Local emission light is normally output with a wavelength that matches a channel grid determined by the International Telecommunication Standardization Sector (ITUT). The wavelength of local emission light is desirably the same as the wavelength of reception light, but there are small wavelength differences therebetween in actual devices. Deterioration of reception quality caused by those wavelength differences is coped with by electrical correction processes by digital signal processors (DSPs) in receivers to thereby make sure that required transfer performance is realized.
In recent years, techniques have been proposed in order to attempt to further expand transfer capacity by utilizing, as the wavelength band of optical signals, the conventional (C) band, the long (L) band, or short (S) band, for example. Such a multi-band WDM system using a plurality of wavelength bands uses wavelength converters that may, for example, convert the wavelength of an optical signal into a different wavelength, and perform wavelength-conversion of C-band multiplexed light into L-band or S-band multiplexed light, to thereby achieve high-capacity transfer.
The related art includes techniques disclosed in Japanese Laid-open Patent Publication No. 2000-75330, Japanese Laid-open Patent Publication No. 2004-348158, International Publication Pamphlet WO 2012/153856, and the like, for example.
When converting the wavelength of received signal light into a different wavelength, a wavelength converter may, in some cases, convert the wavelength into a wavelength that is different from a targeted wavelength after conversion, due to various causes such as changes in environmental temperature or variation in characteristics of optical fibers. Such a wavelength mismatch in some cases makes the wavelength of reception light received at a receiver greatly different from the wavelength of local emission light. For example, if the wavelength of reception light far exceeds a channel grid next to corresponding local emission light, it may become difficult to electrically correct the reception light received on the side of a receiver, and reception of the reception light may become difficult.
In view of the above-mentioned circumstances, it is desirable to provide an optical transfer system or the like that may suppress deterioration of quality of reception light that is received on the side of a receiver.